Research Highlights Transport and Exchange Systems

A hydrophobic membrane with nanopores for highly efficient energy storage

Storing fluctuating and delivering stable electric power supply are central issues when using energy from solar plants or wind power stations. Here, efficient and flexible energy storage systems need to accommodate for fluctuations in energy gain. Scientists from the DWI, RWTH Aachen University and Hanyang University in Seoul now significantly improved a key component for the development of new energy storage systems.

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Microfluidic colloid filtration

Filtration of natural and colloidal matter is an essential process in today’s water treatment processes. Membrane fouling is the most substantial problem in membrane filtration: colloidal and natural matter build-up leads to an increasing resistance and thus decreasing transport rate.We present a method to follow filter cake build up as well as transport phenomena occuring inside of the fouling layer. The microfluidic colloidal filtration methodology enables the study of complex colloidal jamming, crystallization processes as well as translocation at the single particle level.

a) Tortuous path of a small, fluorescent tracer particle. The image is a stack from 600 individual microscope images. The total travel time for the observed particle is 12.52 s.
b) Tracked path of the fluorescent tracer particle with regions of short range order I, II and III. Speed color coded path through the partly crystalline bed.

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Fingerprinting the selectivity of ion-exchange membranes

Ion-exchange membranes with enhanced selectivity for monovalent ions are relevant in a variety of applications: they are used for water softening, as key components for fuel cells or in renewable energy conversion. Using electrochemical impedance spectroscopy (EIS), Prof. Dr.-Ing. Matthias Wessling and his team found a new way to fingerprint selectivity and investigate the competitive transport of monovalent and divalent ions through microgel-modified cation exchange membranes.

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Supramolecular nanofibres for efficient removal of pollutants from water

Adding nanofibres to traditional filtration substrates significantly enhances the filtration performance. Scientists from DWI and Bayreuth University developed a new process for in-situ generation of nanofibers within fibrous or granulated scaffolds. This process is based on supramolecular self-assembly of small molecules.

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Enzymatically Active Ultrathin Pepsin Membranes

A synthetic material can easily be functionalized by embedding biological molecules. Together with colleagues from the University of Twente, Prof. Dr.-Ing. Matthias Wessling (DWI and Aachener Verfahrenstechnik, AVT) and his team synthesized an ultrathin membrane containing the digestion enzyme pepsin. Using this membrane and its enzymatic activity, the scientists can degrade proteins and, at the same time, remove the resulting waste products.

Pepsin membrane formation. The pepsin membrane is positioned atop a porous PAN support, which is represented by the white area. The pepsin membrane consists of pepsin molecules that are randomly cross-linked by trimesoyl chloride, forming amide bridges between the pepsin molecules. The pepsin layer simultaneously acts as the enzymatic surface
and the membrane sieve.

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Ion transport through (POly)electrolyte multi-layers

Ion transport is one of the essential processes that runs in our body, billions of times per day. It also plays a role in a large number of technical processes. However, so far scientists have not been able to simulate the ion transport in its full complexity. Prof. Dr.-Ing. Matthias Wessling (DWI and Aachener Verfahrenstechnik, AVT), Robert Femmer (AVT) and Prof. Dr. Ali Mani (Stanford University) now created a software framework that provides new insights into the transport of ions through membranes.

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Efficient gas-liquid contact in microfluidic devices

Membranes facilitate bubble-free contacting of a gas and a liquid stream. The concentration gradient of the gas component across the membrane promotes the dissolution of gas in the liquid stream. While this process facilitates dissolution of gas in a liquid, it also leads to concentration polarization, thereby preventing efficient gas transfer. Prof. Dr.-Ing. Matthias Wessling and his team developed a microfluidic gas liquid-contacting chip that significantly improves gas transport across the membrane.

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A new method for continuous water desalination

Water is essential for living. Many countries worldwide face water scarcity. Thus, the development of environmentally friendly and cost effective technologies for the desalination of water is a major challenge for researchers worldwide. Researchers at DWI and Aachener Verfahrenstechnik developed a novel system for the continuous desalination of water.

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Rapid prototyping of membranes for microfluidics

Rapid prototyping, additive manufacturing and solid free-form fabrication are synonyms for a technology, which translates virtual computer generated designs into real physical architectures and structures. Rapid prototyping offers complete freedom of design in all three dimensions providing access to novel unprecedented shapes and geometries. Taking this advantage, we developed two methods for the production of three-dimensional polydimethylsiloxane (PDMS) membranes applied as gas-liquid-contactors in a microfluidic setup.

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For decades, electrodialysis using ion exchange membranes has been a mature membrane desalination technology. Like all other membrane processes, concentration polarization also hampers the use of ion exchange membranes at higher current densities, and thus limiting mass flux is reached, beyond which no further mass transfer is theoretically possible. Peculiar to ion exchange membranes, however, so-called overlimiting current can be realized in electrodialysis.

We were now able to prove that controlled adsorption of polyelectrolyte layers at the interface between ionic solution and membrane surface triggers a significantly earlier onset of electroconvection at lower polarization potential. We further proved that their regio-selective adsorption imposed by a soft-lithography molecular printing transfer is even more effective, and induces macroscopic electro-osmotic chaotic fluid instabilities.

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In the present work, we applied thermo responsive microgels in membrane technology. The microgels were infiltrated into the porous structure of a polymer membrane. The microgels were retained by the membranes and adhered either on the surface or in the porous structure. The resulting membranes showed thermo responsive behavior for the pure water flux, but also for the retention of humic acids. With increasing temperature the microgels size decreases and thus the membrane pore opens.

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Microtubes made of carbon nanotubes

Carbon nanotube (CNT) is a pseudo one-dimensional material that can be considered as a cylinder made of rolled graphene sheet with the diameter of nanometer scale and length-to-diameter ratio of more than 1000. CNT combines exceptional mechanical stability, electrical conductivity and surface area. Carbon nanotubes are recognized today as a superior material for preparation of electrodes for electrochemical reactors. The outstanding surface properties make them highly promising for the fabrication of membranes that are highly effective in gas and water separation processes. The research group of Prof. Dr.-Ing. Matthias Wessling recently developed microtubes made of carbon nanotubes.

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On the droplet formation in hollow-fiber emulsification

Hollow-fiber emulsification is a recent development that allows a continuous production of narrow-dispersed droplets with tunable diameter using hollow-fiber membranes. Narrow-dispersed droplets are of particular interest for their use as templates to create particles such as responsive microgels or drug-delivery capsules with defined and homogeneous sizes. In this publication, we studied the droplet formation process inside the membrane using computational-fluid dynamics and comparing these results with experimental data obtained by optical analysis of the formed droplets inside and outside the hollow-fiber membrane.

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Nanofiber filters for sterile water

The market for water filtration grows continuously. Scientists from Aachen and Bayreuth were now able to develop a new method for water sterilization that is only half as expensive as filters made of conventional nonwovens from melt-blown fibers.

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Water treatment with highly efficient textile filters

Wastewater treatments with active coal cannot always remove all organic components, such as drugs. Hence, these components can easily end up in rivers and ground water. Scientists from DWI and Hochschule Niederrhein developed new, cheap adsorbent materials for textile filters.

Application area: Water treatment; drinking water systems for trains or planes and for households that are not connected to a drinking water supply.

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Novel materials for redox-flow-batteries

Energy-efficient and cheap energy storage systems are one of the central challenges of a future based on fluctuating energy sources. DWI and several partners from academia and industry are working in a joint project, called tubulAir±, to develop a new type of redox flow battery that can tackle that challenge - a tubular vanadium air redox flow battery. This tubular redox flow battery uses a vanadium electrolyte to store electrical energy and humid air as a reaction partner during charge and discharge, allowing a compact and cheap universal battery for energy applications.

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New methods to characterize the morphology of proton exchange membranes for fuel cells

Fuel cells hold great promise as an efficient and clean energy-conversion technology and will encourage the development of an alternative energetics. Over the past decades, there has been great interest in proton-conducting polymers and their application in polymer electrolyte membrane fuel cells. Many studies have been conducted on the development of new proton exchange membranes and understanding of proton-transport mechanisms.

For the first time the diffusional kurtosis used recently for investigation of brain structure by NMR imaging is introduced to characterize the heterogeneity of the channel architecture for the water transport in proton exchange membranes.

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Novel conductive materials with high charge carrier density

Storage of energy is one of the major challenges of the energy turnaround towards a sustainable and renewable but also variable supply of energy. To overcome this limitation, novel materials with extremely large surface area and hierarchically structured porosity will be developed to obtain more efficient electrodes for supercapacitors and batteries. Polyacrylo nitrile can be spun to achieve very thin fibers of only a few hundred nanometers in diameter.